1. Field of the Invention
The present invention relates to an image forming apparatus including a fixing unit configured to fix a toner image on a sheet and a cooling unit configured to cool the sheet having the image fixed thereon.
2. Description of the Related Art
A conventional electrophotographic type image forming apparatus, such as a copying machine, a printer, or a facsimile apparatus, transfers a toner image on a sheet, applies heat and pressure on the sheet having the toner image transferred thereon to fix the toner image by using a fixing device, and after that, discharges the sheet having the fixed image to the outside of the apparatus. A sheet may be curled during an operation for fixing a toner image. Degree of a curl of a sheet may differ according to a sheet type, temperature, humidity, and density of an image formed on the sheet.
If a large curl may occur on a sheet during fixing, the sheet may catch in a joint or a hole of a conveyance guide, which may cause a jam. In addition, if a very large curl has occurred on a sheet, a sheet alignment failure may occur during post-processing, such as stapling or folding. In this case, the resulting post-processed product may not have a sufficiently good appearance. In order to solve the above described problem, Japanese Patent Application Laid-Open No. 10-198080 discusses the following method. More specifically, in an image forming apparatus according to this conventional method, a curl amount changing unit, which is provided downstream of a fixing device and configured to change the amount of curl of the sheet, is calibrated according to the type of the sheet and the image density to reduce the amount of the curl.
Conventionally, the orientation and the amount of a curl that may occur at a fixing device according to a predetermined parameter, such as the sheet type or the image density, is changed so that the degree of the sheet curl is reduced by calibrating a curl amount changing unit according to a predicted value or an experimental value obtained by a previously executed experiment. However, the following problems may arise in the conventional method for calibrating a curl amount changing unit.
More specifically, in the conventional method, if image forming is executed at a delayed timing due to delayed image processing on data that has been externally input, the conveyance of a sheet is suspended. In this case, if a sheet having a fixed image is stopped in a conveyance path and if a cooling fan, which is a cooling unit, for fixing a toner image transferred on a sheet exists at a location at which the sheet has been stopped, the degree of the curl may vary. In other words, the degree of a curl may differ according to whether the sheet has been stopped at the cooling unit or not.
To paraphrase this, in the conventional image forming apparatus, a setting value, which is set to the curl amount changing unit, is not set based on a premise that the amount of curl may vary due to suspension of sheet conveyance. Accordingly, if a sheet is stopped at the cooling unit, the curl that has occurred on the sheet may not be set off by the curl amount changing unit to a sufficiently small dimension. Further, in this case, the curl amount changing unit may adversely cause the sheet to curl or increase the curl on the sheet.
If the sheet curl amount cannot be appropriately controlled or changed, a transfer failure may occur when a sheet enters a transfer unit, which transfers a toner image on the sheet, to execute image formation on a second side of the sheet after forming an image on a first side of the sheet. More specifically, if the amount of curl has not been appropriately changed when it is required for the curl amount changing unit to change the curl amount small enough for the sheet to enter the transfer unit in a downward-bent curl state (i.e., in a state where the sheet has a curl having an upward convex shape), the sheet may enter the transfer unit in an upward-bent curl state due to the failure of appropriately changing the curl amount.